If my site has 100k internal pages for instance. And I want to link to another site with 100k internal pages. If every one of my individual pages links to a unique page on the other site, and that page links right back. So basically what I will have is 100k total links exchanged but only 1 exchange / page. Will that be helpful for SEO purposes?
You may see a small surge in your serps but it will not last long. It is better to have many links from many sites than many links from same site.
That is such an unnatural way of linking that I can't believe the google algorithm won't detect it, see it as 'spammy' and disregard most of the links.
It's bad idea. It would be much better to exchange only one link with many sites. However personally I do not like any reciprocal link exchanges.